How much is our intuition about sauropod mass worth?

June 5, 2014

Back in 2008, when I did the GDI of Giraffatitan and Brachiosaurus for my 2009 paper on those genera, I came out with estimates of 28688 and 23337 kg respectively. At the time I said to Matt that I was suspicious of those numbers because they seemed too low. He rightly told me to shut up and put my actual results in the paper.

So evidently I have a pretty strong intuition that Brachiosaurus massed somewhere in the region of 35000 kg and Giraffatitan around 30000 kg. But why? Where does that intuition come from?

I can only assume that my strongly held ideas are based only on what I’d heard before. Back when I did my 2008 estimate, I probably had in mind things like Paul’s (1998) estimate of 35000 kg for Brachiosaurus, and Christiansen’s (1997:67) estimate of 37400 for Giraffatitan. Whereas by the time the Benson et al. paper came out I’d managed to persuade myself that my own much lower estimates were right. In other words, I think my sauropod-mass intuition is based mostly on sheer mental inertia, and so should be ignored.

The underlying problem here is that most of us have no useful intuition about what any animal weighs. Show me a hippo or even a cow and I’d be hard-pressed to tell you its mass within, say 20%, maybe more. So what chance do we have of applying that intuition to fossil animals?

I’m guessing that, since most any tetrapod can vary in weight by at least fifty percent, all your intuitions are both correct and incorrect. Animals, in the wild, can starve, be well fed, be dehydrated, be well hydrated and so on. So, weight estimates by bone diameters, by displacement figured by models, by estimating the mass of various parts…lungs lighter, bones heavier, etc., are likely all within the ballpark of actual creature masses.

I certainly agree with that as a principle, Bryan. But the degree of variation in mass estimates for the same individual is sometimes so extreme that there’s no question some of them have to be complete nonsense. For example, as I showed in my sauropod history paper, published estimates for the single Giraffatitan brancai individual MB.R.2181 (= HMN SII) have varied from 13618 kg to 78258 kg, stopping at all stations in between.

Very interesting. I wonder if there is a trend resulting from how we imagine and draw dinosaur. Indeed we have heavy weight Giraffatitan in the beginning of the 20th and in the ninety skin wrapped sauropod. Does the estimation of Giraffatitan follow a temporal trend with a decrease till recently?

I think that volumetric approaches are needed first before we can develop single-bone techniques. But these volumetric approaches absolutely need to be ground truthed in a wide sample of “knowns” of diverse body form.

78258 kg (Colbert’s estimate?) doesn’t even make sense for HMN XV2, let alone the smaller SII. Volumetric models are more predictable than bone-circumference techniques, but not all models are good. I’ve seen both extremely anorexic and extremely obese reconstructions for several sauropods. You models are only as good as your artist and their scientific knowledge.

Sounds like it’s time to get rid of the “one estimate fits all” lunacy. IF we took a human skeleton, let’s say, an adult male, five foot ten, mesomorphic build and tried to say the guy weighed one hundred seventy pounds, then we’d probably realize that we are ignoring the fact that weights in a single individual can vary all over the map. As Mike said, some of the estimates are ridiculous, but what I myself find way more ridiculous is trying to estimate an exact weight for a fossil beast when getting exact weights for a living, breathing being is a crap shoot. Take your own self, for example; weigh yourself in the morning, afternoon, and evening and see if there’s any variance. Go on short commons for two weeks and tell me there’s no difference. Pig out when food is available and tell me there’s no difference.